High voltage composite insulators typically comprise an insulating rod (mostly made of glass fiber reinforced hard plastic material), which absorbs the mechanical loads, a jacket and ribs made of insulating plastic material (mostly a plastic material with hydrophobic or contaminant repellant surface, like silicon rubber) is disposed above said rod a, and they comprise end fittings (mostly made of metal) permanently connected to the rod, which are used as mounting devices.
Corona discharges quite frequently occur in a high voltage insulator under voltage. Such a corona discharge can have an eroding effect upon the insulating jacket. Thus, it can pit the surface of the insulating jacket in particular proximal to the end fittings, because the field strength is generally very high in that area, and thus the corona discharge can reduce the service life of the insulator.
It is a common countermeasure to form the electrical field at the end fitting by means of so-called corona rings made of conductive material, mostly metal or semi-conductive material, mostly plastic intermixed with conductive particles, which corona rings are often mounted at the end fitting, so that the field strength is reduced. This reduces the propensity for a corona discharge, or it reduces at least the concentration of the corona discharge at the surface of the insulator jacket proximal to the end fitting.
Alternatively, the printed document U.S. Pat. No. 6,984,790 B1 describes a corona shield made of metal, which is assembled from two metal half shells over the insulator at the interface between the rod and the end fitting. A cavity formed by the metal half shells is then filled by a sealant compound.
As discussed, also semi-conducting corona shields are known, e.g. from U.S. Pat. Nos. 6,388,197 B1 and 4,355,200.
Such field forming measures, however, are rather complex, and additionally they are often not sufficient for actually preventing critical corona discharges. Field forming is therefore omitted in many composite insulators. It has already been suggested for such insulators to better protect the insulating jackets close to the rod against erosion by means of insulating material with greater wall thickness. Namely through the greater wall thickness, it takes longer until corona discharge induced erosion permeates the insulator wall in said particularly exposed portion. Thus, the service life reducing effect of the corona discharge is compensated by said measure. Furthermore, the corona shield can also be used for sealing the interface between rod and end fitting, e.g. against rain water. An embodiment of a composite insulator with such a corona shield made of insulating material is known from the printed document U.S. Pat. No. 3,898,372.
However, the composite insulator known from U.S. Pat. No. 3,898,372 is not to be considered optimum with respect to service life and manufacturing complexity. Thus, it is the object of the present invention to provide a composite insulator with a long service life, which can still be produced with rather low complexity. This also includes providing a coronal shield, which facilitates producing such insulator.